In a mobile communication system, high-speed data communication is required to cope with a mobile multimedia service such as Internet connection. Retransmission control is used as a means for efficient transmission of packet data for high-speed data communication. The retransmission control (ARQ: Automatic Repeat Request) is a technique applied when an error is detected in a received packet on the receiving side, so as to request the transmission side to transmit the packet concerned.
Further, there has been put into practical use the hybrid ARQ (H-ARQ), in which ARQ is combined with an error correction code (FEC: Forward Error Correction). At present, as a third-generation (3G) radio communication system, W-CDMA is widespread, and H-ARQ is also adopted to HSDPA (High-Speed Downlink Packet Access) by which high-speed (14 Mbps maximum) W-CDMA data communication is attained.
IN H-ARQ, the receiving side performs error correction processing prior to error detection, and when it is decided that the reception is successful (without an error), an ACK (decision of normality) is transmitted to the transmitting side, while when a reception failure (inclusive of an error) is decided, a NACK (decision of abnormality) is transmitted to the transmitting side, so that the retransmission of the packet data is requested. The transmitting side retransmits the packet data corresponding to the received NACK.
Further, in H-ARQ, the receiving side stores into a buffer the packet data in which an error is detected, so as to perform packet combination processing to combine with the retransmitted packet data. By that the receiving side combines with the packet data stored in the buffer when receiving the retransmitted packet data, the reception quality is improved, and as the number of retransmissions increases, the degree of improvement is increased, and the rate of successful packet data reception is increased.
Further, in the above retransmission processing, the transmitting side receives ACK/NACK information from the receiving side after the data for one radio frame is transmitted from the transmitting side. By using, as a unit of processing, a plurality of radio frames transmittable within a time (RTT: Round Trip Time) before the completion of the retransmission decision of the radio frame data concerned, the retransmission decision and the retransmission are performed on the basis of each radio frame.
Usually, as retransmission management information, a process ID/new data identifier (1 bit) is assigned to each radio frame in RTT, and retransmission management is performed on the basis of each process ID. Namely, at the time of data transmission, the retransmission management information is added to a control channel to be transmitted in an identical frame, so that the receiving side is informed. On the receiving side, combination processing, decoding processing and reception decision of the retransmitted data are performed on the basis of each process ID. The reception decision result is reported to the transmitting side as ACK/NACK information, as described above. Here, because the transmitting side already knows the process ID of the data having been transmitted before the RTT in which the ACK/NACK information is to be received, the notification of the process ID information together with the ACK/NACK information is unnecessary. Further, on the transmitting side, when the transmission data (data in a retransmission buffer) managed on the basis of each process ID is switched from the retransmission data to the next new data (the process ID is released when the retransmission of the transmission data managed by the process ID concerned is completed, or when it is decided that the retransmission failed even after reaching the maximum number of retransmissions. The released process ID is then assigned to the next new transmission data, and thereby the retransmission control for the next new transmission data is started), the notification to the receiving side is performed by the transmission of the new data identifier with an inverted logic. On the receiving side, by the detection of a change of the new data identifier, the retransmission processing of the process ID concerned is completed, and the start of reception of the next new data is identified.
Further, the receiving side once stores a plurality of radio frame data, of which reception after the retransmission processing is decided to be successful, into a sequence control buffer, and performs processing (sequence control processing) to restore the sequence of the radio frame data having been changed by the retransmission processing. In regard to the above sequence control buffer, there is required an extremely large size enough to store the entire decoded data of the number of radio frames transmittable and receivable during the time of (RTT×the maximum number of retransmissions). Because of a limited hardware scale on the receiving side, there is an upper limit of a mountable memory size, and accordingly, in the present state, the upper limit of the maximum number of retransmissions in the apparatus is determined by the above limitation.
FIGS. 1 through 4 are diagrams illustrating a variety of exemplary states in the retransmission processing of the related art. FIG. 1 illustrates a case under a good reception quality state, in which no retransmission occurs. FIG. 2 illustrates a case under a reception quality state being deteriorated to a certain extent, such that retransmissions occur in several frames, and however, the entire reception is successful. Also, FIG. 3 illustrates a case under a state that the reception quality is further deteriorated, such that the retransmissions fail even after reaching the maximum number of retransmissions. FIG. 4 illustrates a case under the state of the reception quality exceedingly deteriorated as compared to FIG. 3, such that the ratio of the radio frames, of which retransmissions fail even after reaching the maximum number of retransmissions, increases.
More specifically, in FIG. 1, RTT=8 radio frames, and transmission data Dn (n=1, 2, 3, . . . ) are transmitted from the transmitting side in each radio frame. On the receiving side, the transmission data Dn are received after 3 radio frames. The receiving side receives each transmission data Dn entirely normally through a single reception, and based on ACK from the receiving side, the transmitting side further transmits a new transmission data Dn. As in the case illustrated in FIG. 1, when no retransmission occurs, the entire 8 radio frames are assigned to the transmission of new data, and transmission under the maximum rate is carried out accordingly.
In FIG. 2, retransmissions occur in regard to transmission data D3, D4 and D7. In regard to the transmission data D3 and D7, normal reception is attained by the first retransmission (the second transmission), while in regard to the transmission data D4, normal reception is attained by the second retransmission (the third transmission). When the retransmission of the transmission data Dn is performed, the number of radio frames assigned to the transmission of new data is reduced by that amount, and therefore, the transmission rate is decreased as compared to the case of FIG. 1. Further, when the maximum number of retransmissions is assumed to be 5 for example, in FIG. 2, the entire transmission data Dn are received normally by means of the retransmissions within the maximum number of retransmissions.
In FIG. 3, normal reception is not attained even when the number of retransmissions of the transmission data D4 reaches the maximum number of retransmissions (5), and therefore, a process ID 3 of the radio frame having been assigned to the transmission data D4 is released, and a new transmission data D11 is assigned to the radio frame concerned. Also, in regard to the transmission data D3, normal reception is attained at the time of the maximum number of retransmissions. By this, a radio frame process ID 2 having been assigned to the transmission data D3 is released, and a new transmission data D17 is assigned to the radio frame concerned. Here, transmission data other than D3 and D4 are received normally in one retransmission or less.
In FIG. 4, normal reception is not attained even if the number of retransmissions of the transmission data D4, D5 reaches the maximum number of retransmissions (5), and therefore, each process ID 3, 4 of the radio frames having been assigned to the transmission data D4, d5 is released, and new transmission data D9, D10 are assigned to the radio frames concerned. Further, in regard to the transmission data D3, normal reception is attained at the time of the maximum number of retransmissions. By this, the radio frame process ID 2 having been assigned to the transmission data D3 is released, and a new transmission data D11 is assigned to the radio frame concerned. Also, in regard to the transmission data other than the transmission data D3, 4, 5, there are relatively a large number of retransmission failures (NACK), and retransmission is repeated accordingly. Thus, as compared to the case illustrated in FIG. 3, there are a smaller number of radio frames assigned to the new data.
Additionally, in the patent document 1 illustrated below, instead of managing data retransmission on a packet-by-packet (data-by-data in the unit of a radio frame) basis as illustrated in FIGS. 1 through 4, there is disclosed retransmission control in which a frame period to perform retransmission is set in advance, and when the above frame period arrives, only a packet(s) on which ACK has not been received is retransmitted.
[Patent document 1] Japanese Laid-open Patent Publication No. 2002-261774.